The present invention relates to channel electron multipliers, both single-channel and multiple-channel types, such as are used for detecting low energy electrons and other charged particles or radiations and producing amplified outputs thereof.
Channel electron multipliers are commonly used to detect any charged particles or radiation capable of producing secondary electrons. Some applications to which they have been put are for detecting electrons, ions, alpha-particles, beta-particles, X-rays and ultra-violet photons; and some of the fields in which they have been used are X-ray and ultraviolet astronomy, filed ion microscopy, mass spectrometry, and ultra-violet spectroscopy.
The conventional single channel electron multiplier consists of a glass tube under vacuum, the inner surface of which tube is coated with a semi-insulating or resistive material capable of emitting secondary electrons when struck by an incident electron or other charged particle or radiation. The resistivity of the coating is about 10.sup.8 to 10.sup.13 ohms between the electrodes so that a strong electric field can be generated. The ratio of channel length to diameter for optimal multiplication is about 50:1. A voltage is applied between the ends of the tube to produce a fairly uniform electric field within it. Electrons which enter the tube are accelerated by the electric field until they strike the resistive coating, causing secondary electrons to be emitted, which in turn are accelerated and strike the resistive coating to cause further secondary electrons to be emitted. The gain of such multipliers varies between 10.sup.3 and 10.sup.8.
The gain of the multiplier is independent of its absolute dimensions, and therefore these dimensions may be made extremely small. A large number of such channel multipliers may therefore be mounted in parallel and used to detect radiation in two dimensions. One form of such array of multiple-channel multipliers, called a channel plate, is made by fusing a plurality of straight channels together to form a honeycomb-like cross-section, polishing the input and output faces of the plate, and evaporating a thin metallic coating on these polished faces at an oblique angle, such that the coating connects the glass interstices at the end of the channels but leaves the channel tubes open. A typical channel plate, composed of 40 micron diameter channels, may have an open area of 60% and a thickness of 2mm.
Most of the research efforts in this field have been concentrated on the development of the single channel multiplier, mainly to improve its functioning and to make the channel diameter as small as possible. Extended experimental and theoretical studies have been made on different secondaryemission materials and on the influence of the various parameters governing the properties of the channels. Outstanding high gain, narrow pulse height distribution, and substantial channel uniformity having a diameter of a few microns have been attained.
Very low noise and low dark current devices, such as channeltrons and spiraltrons, have been fabricated and used.
On the other hand, less attention has been paid to the construction of channel plates, although they have found applications in space-exploration experiments and in field-ion microscopy. Channel plates have been built in the manner described earlier with a position resolution as high as 50 lines per mm. However, the effective area of these devices is very small (a few tenths of an in. sq.), and therefore in the electro-optical systems in which they have been used (e.g. the vidicon), the image has to be first reduced considerably in size, resulting in a low gain and a low uniformity.
Another serious drawback of the prior devices is their high cost, since their fabrication, particularly the multiplechannel multiplier, involves a complicated and costly procedure.